turn CAS into compare-and-swap instead of compare-and-set: make it return the old value

6 jobs for ci/for_proph in 13 minutes and 48 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #34423
fp
build-coq.8.10.dev

00:12:22

passed #34427
fp
build-coq.8.7.2

00:06:24

passed #34426
fp
build-coq.8.8.2

00:07:20

passed #34425
fp-timing
build-coq.8.9.0

00:06:32

passed #34424
fp
build-coq.8.9.1

00:07:55

passed #34422
fp
build-coq.dev

00:13:30